purchase_order_view.xml

<?xml version="1.0"?>
<openerp>
    <data>
        <record model="ir.ui.view" id="purchase_order_form_package">
             <field name="model">purchase.order</field>
             <field name="inherit_id" ref="purchase.purchase_order_form" />
             <field name="arch" type="xml">
                <xpath expr="//field[@name='product_qty']" position="before">
                    <field name="product_qty_package"/>
                    <field name="package_qty"/>
                    <field name="indicative_package" invisible="1"/>
                </xpath>
                <xpath expr="//field[@name='price_unit']" position="after">
                    <field name="price_policy"/>
                </xpath>
             </field>
        </record>
    </data>
</openerp>